Thin film resistors are typically manufactured by depositing resistive films of various alloys onto a non-conductive substrate. Typically, an aluminum oxide or aluminum nitride ceramic substrate is used, but other substrate materials can be used including, but not limited to, glass, diamond, ruby and metallic substrates having a non-conductive coating. The deposited films range in thickness from a few hundred Angstroms to several thousand Angstroms, depending on the desired sheet resistance.
Formation of the resistive films can be accomplished through a range of processes including plasma enhanced chemical vapor deposition (PECVD), chemical vapor deposition (CVD) or physical vapor deposition (PVD), with PVD being the most typical method used for thin film resistor manufacturing. Film deposition with a PVD process is typically performed in a vacuum environment. Deposition rates for a PVD process vary between about 0.005 to about 0.2 micrometers per minute for typical materials used in resistor manufacturing.
In thin film resistor design, the resistor value may be determined through the combination of the sheet resistance of the deposited film, measured in Ohms/Square, and the number of squares defined by the resistor geometry. For example, a 100 ohm resistor can be manufactured using a 50 ohm/square film and a design that has a two (2) square resistor geometry.
While PVD thin film technologies are effective at manufacturing precision resistors at nominal values of 10 ohms or above, the feasibility of creating lower resistance values, such as those in the range of about 10 ohms or less, or about 1 ohm or less, diminishes rapidly due to limitations in achieving the necessary film thicknesses in a practical and commercially reasonable time period. Accordingly, there is a need for a process for manufacturing a resistor that is faster than known techniques, yet is still precise, efficient and cost effective.
Thermal spraying is a process whereby heat is used to soften a material such as a metal or a ceramic, and then particles of the softened material are propelled, such as by a gas, onto a substrate to be coated. Other forms of energy, such as kinetic energy, may be used to accelerate the particles to a velocity whereby plastic deformation occurs when the particles impact the substrate. The particles form a dense coating/layer on the substrate as the particles agglomerate. The material to be thermally sprayed is sometimes referred to as the “feedstock.” An example of equipment used for thermal spraying, and equipment that may be used for making a thin film resistor according to the present invention, is the Kinetic Metallization: Production Coating System, KM-PCS, offered by the Inovati Company of Santa Barbara, Calif.
Thermal spraying techniques can deposit metals at rates several times faster than PVD, PECVD or CVD processes generally used to form thin film resistors. For example, thermal spraying can deposit materials at a deposition rate of about at least 10 micrometers per minute. The high deposition rate of thermal spraying allows low value resistors to be made at a more competitive cost than known techniques. In contrast to thermal spraying, a PVD process cannot achieve the thicknesses required in a practical time period.
Thermal spraying, while typically performed in ambient conditions, can also be performed under a range of environments or conditions to control the oxide level and, to some extent, the structure of the thermally sprayed material. An advantage to spraying in ambient conditions is a reduction in processing time due to lack of required pump down time of a vacuum or other environmentally controlled system. Thermal spraying technologies available in the industry vary by the method of applying material, for example, the type of energy used and by the type of material used as the feedstock.
The present invention provides a means to address the time constraint and cost problems associated with deposition of resistive elements in thin film resistors using known techniques, by the application of thermal spraying technologies to the manufacture of thin film resistors.